Absorption heat pump for extreme operating conditions

ABSTRACT

An absorption heat pump with a system for improving its efficiency under extreme conditions by bleeding off refrigerant downstream of the condenser and mixing it with the rich solution after this latter has been at least partially heated by the absorber and before it is fed into the desorber.

The present invention relates to a heat pump for extreme operating conditions.

In known heat pumps used for heating, the facility to provide high temperatures is a merit because such pumps can then replace traditional boilers without modifying the systems in which they are inserted. These traditional systems present for example a burner associated with a heat exchanger through which water is pumped to feed one or more radiators or convectors.

Even in low or medium temperature heating systems, known climatic curve controllers require the temperature of the water fed to the radiators (or radiant panels or convectors) to be increasingly higher as the temperature of the external environment decreases.

One of the difficulties of widely promoting heat pumps in general is precisely the impossibility of bringing the water circulating within the fixed system to a temperature higher than 65° C. Under conditions close to that temperature the effective efficiency of the heat pump falls off drastically and is considerably different from the theoretical efficiency.

The need to obtain relatively much higher water temperatures (higher than 65° C.) is greatest essentially in two cases: when the external temperature is very low and when hot water is required for domestic use.

As already stated, under these conditions the heat pump efficiency falls to very low levels.

Solutions have been implemented which enable high temperatures to be obtained in such conditions; however these solutions use burner power modulation which essentially decreases the pump power itself and is unacceptable.

An object of the present invention is to provide a heat pump able to supply high temperature water to a fixed heating system or to a domestic water generation system while maintaining high efficiency, preferably while maintaining the generator at its maximum power.

This and other objects are attained by a heat pump formed in accordance with the technical teachings of the accompanying claims.

Further characteristics and advantages of the invention will be apparent from the description of a preferred but non-exclusive embodiment of the heat pump, illustrated by way of non-limiting example in the accompanying drawings, in which the single figure shows a simplified scheme of the heat pump of the present invention.

With reference to said figure, this shows a heat pump indicated overall by the reference numeral 1.

It operates with a cycle using as refrigerant a first fluid (in this specific case ammonia), which is absorbed in a second fluid (in this case water). The absorption heat pump comprises a conventional generator 2 or desorber presenting a finned gas burner 35, which feeds a conventional plate column 36. The plate column 36 is connected to a rectifier 33, described hereinafter. The vapour outlet of the generator is connected via a rectifier and a first line 3 to a condenser 4 of conventional type, positioned in heat exchange contact with a transmission fluid which feeds the heating plant. This fluid is typically water fed into the plant by a pump, not shown.

A countercurrent heat exchanger 34 is provided downstream of the condenser 4 in a second line 6 connecting the condenser to an evaporator 34 via a lamination valve 5, to exchange heat with the vapour circulating through a third line 8 connecting the evaporator 7 to an inlet 10B of an absorber 10. A further lamination valve 36 is provided upstream of the heat exchanger 34.

As already stated, an evaporator outlet 7B is connected by a third line 8 to an inlet 10B for vapour from said first fluid into the absorber 10, and specifically into a mixing zone 9.

The absorber 10 comprises a rich solution outlet 10C (ammonia absorbed in water) connected to a heat exchanger 13 in heat exchange contact with the transmission fluid of the heating plant.

An outlet 13B of the heat exchanger is connected to the suction side of a conventional pump 14, the delivery side of which is connected via a fourth line 15 to an inlet 16 of a circuit 16A, 16B in heat exchange contact with the absorber 10.

The fourth line 15 is in heat transmission contact with the rectifier 33 from which the rich ammonia solution subtracts heat to facilitate condensation of water vapour.

The circuit 16A, 16B subtracts heat from the absorber to hence transfer it to the rich solution originating from the pump 14 before being fed into the generator 2. This circuit is divided into two parts only for reasons of description. In this respect, in the first part of the circuit 16A the rich solution rises in temperature, while in the second part 16B the ammonia present in the solution begins to evaporate (at the pressure present in the circuit 16A, B) to essentially anticipate the work done by the generator 2. That part of the absorber involved with the circuit part 16B is commonly known as a GAX cycle.

A fifth line 18 extending from the heat exchanger 10 connects an outlet of the circuit 16A, 16B to an ammonia enriched solution (plus ammonia vapour) inlet 2B of the generator 2.

At the generator base, in proximity to the burner 35, an outlet 2C is provided from which a poor ammonia solution is directed, via a sixth line 19 provided with at least one lamination valve 30, to a poor solution inlet 10A provided in the absorber 10, after yielding heat to the fluids present in the generator in a central portion 2D thereof.

The present invention provides a system for maintaining the top of the desorber plate column “colder” and reducing the rectifier load when high temperatures are required at the heat exchangers 4 and 13. To achieve this, the flow and/or NH₃ concentration of the rich solution entering the generator 2 is increased. This can be done by bleeding off part of the liquid refrigerant leaving the condenser and mixing it with the rich solution line entering the generator, by using the suction effect of a liquid-liquid injector.

Specifically, a point 22 for the introduction (or feed) of condensed vapour (liquid ammonia) into the rich ammonia solution is provided between the inlet 16 of the circuit composed of the first and second part 16A, 16B and the rich solution inlet 2B of the generator.

The introduction point 22 is shown by a full line and indicated by the reference numeral 22A. With this solution the withdrawal line 20 which starts from the withdrawal point 24 advantageously feeds into the venturi 22A shown in the figure. This is positioned in a circuit portion downstream of the first part 16A and upstream of the second part 16B. It is important that the introduction of refrigerant takes place at a point downstream of which there is at least one further heat exchange for the rich solution, in this case with the absorber 10.

Introducing bled refrigerant into the solution flow “costs” in terms of machine power (refrigerant flow to the evaporator). This cost can be minimized to obtain an advantage under certain conditions.

This introduction point is particularly advantageous when located in a point of the circuit 16A, 16B in which the solution present therein has a temperature close to that of the temperature resulting from mixing the two flows, i.e. the refrigerant flow and the solution flow. In this respect, adiabatic mixing of two liquid flows [for example 44% NH₃ in the solution, 99% NH₃ in the refrigerant] results in a flow at a temperature greater than the two inlet temperatures.

This optimum temperature is between 60° C. and 90° C., preferably between 70° C. and 80° C. If the refrigerant bypass flow is for example 10% of the refrigerant, then ammonia concentration in the rich solution can increase by between 2 and 4%. This implies that the GAX regenerator (second portion 16B of the circuit) begins to reboil the solution at a temperature less by 4° C. and 6° C., compared with when the ammonia concentration in the solution is less.

For example, for an ammonia concentration of 44% in the solution, the boiling temperature at 20 bar is 103° C. By increasing the concentration to 47% with the bypass line 20, 20A, 20B, the boiling temperature falls to 97° C. at the same pressure. The vapour regenerated hence “recovers” the expense of the bypass.

This results in a lowering of the desorber column and rectifier temperature by about 10-15° C., with considerable benefits. The result is that for equal evaporator power there is a greater “load” at the condenser (which therefore has to be slightly over-dimensioned).

However there is a lesser load at the rectifier and generator, which work at lower temperature.

This situation becomes very interesting precisely when high (>65° C.) water temperatures are required from the heating plant, or for generating domestic hot water. In this case, conventional heat pumps generate pressures and temperatures which cause the desorber column to “work” at its limit, so bringing the rectifier load to critical levels, and drastically reducing the refrigerant flow fed to the condenser (also because the GAX regenerator at these high pressures does not regenerate refrigerant vapour). Increasing the heat exchanger surfaces does not improve the situation, while at high temperatures the risk of surface corrosion increases.

Bypassing the refrigerant according to the invention increases the rich solution concentrations, so extending system working conditions.

The refrigerant injection or feed takes place preferably by means of a venturi, which enables the refrigerant to be “drawn” into the solution.

However, injection can be effected by any other suitable means.

In addition to comprising a refrigerant non-return valve 32, the refrigerant feed line 20 can also comprise a solenoid valve or the like which completely excluders the bypass line, hence enabling the heat pump to be used in a completely conventional manner.

It has been seen that by introducing the aforedescribed circuit modification, the heat pump operates under a wide variety of conditions, with much higher efficiencies than conventional heat pumps, especially when these conditions are extreme.

Various embodiments of the invention have been described, but others can be conceived by utilizing the same inventive concept. All the described components can be replaced by technically equivalent elements. Moreover the refrigerant and the liquid in which it is absorbed can be chosen at will in conformity with the necessary technical requirements. 

1. An absorption heat pump comprising: a generator or desorber for generating , from a first fluid, vapor fed via a first line to a first condenser in heat exchange contact with a transmission fluid, downstream of the condenser there being provided a second line entering an evaporator, the second line comprising at least a first lamination valve, an evaporator outlet being connected by a third line to an inlet for vapor from said first fluid into an absorber, comprising an absorber outlet for an enriched solution of said first fluid absorbed in a second fluid, the absorber outlet being connected to a heat exchanger in heat transmission contact with the transmission fluid, a heat exchanger outlet of the heat exchanger being connected to a suction side of a pump, a delivery side of the pump is connected by a fourth line to an inlet of a circuit in heat transmission contact with the absorber, a fifth line connecting said circuit to a rich solution inlet of the generator, the generator having a poor solution outlet connected by a sixth line provided with a second lamination valve to a poor solution inlet provided in the absorber, wherein an introduction point of condensed vapor from said first fluid circulating through the circuit is provided between the inlet of the circuit and the rich solution inlet of the generator.
 2. A heat pump as claimed in claim 1, wherein the condensed vapor is withdrawn at a withdrawal point positioned directly downstream of the condenser by a withdrawal line.
 3. A heat pump as claimed in claim 1, wherein a non-return valve is provided in the withdrawal line, between the withdrawal point and the introduction point.
 4. A heat pump as claimed in claim 1, wherein the introduction point is in the form of a venturi.
 5. A heat pump as claimed in claim 1, wherein said introduction point is provided between a first and a second portion of said circuit.
 6. A heat pump as claimed in claim 1, wherein said withdrawal line comprises a valve arranged to exclude the withdrawal line when necessary.
 7. A heat pump as claimed in claim 1, wherein a rectifier in heat exchange contact with the fluid leaving the pump is provided between the generator and condenser.
 8. A heat pump as claimed in claim 1, wherein the sixth line is in heat exchange contact with a central portion of the generator.
 9. A heat pump as claimed in claim 1, wherein the fluids present in the second and third line are brought into heat transmission contact by means of a heat exchanger.
 10. A pump as claimed in claim 9, wherein a further lamination valve is provided at the inlet of the heat exchanger.
 11. A method for improving the efficiency of an absorption heat pump according to claim 1, when under desorber power modulation conditions, comprising the step of: bleeding off liquid refrigerant downstream of the condenser, and mixing the liquid refrigerant with the rich solution after this rich solution has been at least partially heated by the absorber and before the rich solution undergoes further heat exchange with the absorber and is fed into the desorber.
 12. A method as claimed in claim 11, wherein the refrigerant is bled off between the condenser and the evaporator.
 13. A method as claimed in claim 11, wherein the refrigerant is mixed with the rich solution at a point in which the difference between the temperature of the solution before its mixing and the temperature resulting from mixing the solution with the refrigerant is between −5° C. and 5° C.
 14. A method as claimed in claim 11, wherein the introduction point is in a region in which the solution temperature is between 60° C. and 90° C. for the heat pump in which the refrigerant is ammonia and the liquid in which the ammonia is absorbed is water.
 15. A method as claimed in claim 11, wherein said mixing is achieved by a venturi.
 16. A method for improving the efficiency of absorption heat pumps according to claim 1, when under desorber power modulation conditions, comprising the step of: bleeding off liquid refrigerant downstream of the condenser and mixing the liquid refrigerant with the rich solution after this rich solution has been at least partially heated by the absorber and before the rich solution undergoes further heat exchange with the absorber and is fed into the desorber, wherein said bleeding can be excluded, depending on the pump working conditions.
 17. A method as claimed in claim 11, wherein the introduction point is in a region in which the solution temperature is between 70° C. and 80° C., for the heat pump in which the refrigerant is ammonia and the liquid in which the ammonia is absorbed is water. 